Serum albumin binding of palmitate and stearate. Multiple binding theory for insoluble ligands

Fatty acid binding to the human transport proteins FABP3, FABP4, and FABP5 from a Ligand's perspective

Fatty acid binding proteins (FABPs) are a family of amphiphilic transport proteins with high diversity in terms of their amino acid sequences and binding preferences. Beyond their main biological role as cytosolic fatty acid transporters, many aspects regarding their binding mechanism and functional specializations in human cells remain unclear. In this work, the binding properties and thermodynamics of FABP3, FABP4, and FABP5 were analyzed under various physical conditions. For this purpose, the FABPs were loaded with fatty acids bearing fluorescence or spin probes as model ligands, comparing their binding affinities via microscale thermophoresis (MST) and continuous-wave electron paramagnetic resonance (CW EPR) spectroscopy. The CW EPR spectra of non-covalently bound 5- and 16-DOXYL stearic acid (5/16-DSA) deliver in-depth information about the dynamics and chemical environments of ligands inside the binding pockets of the FABPs. EPR spectral simulations allow the construction of binding curves, revealing two different binding states ('intermediately' and 'strongly' bound). The proportion of bound 5/16-DSA depends strongly on the FABP concentration and the temperature but with remarkable differences between the three isoforms. Additionally, the more dynamic state ('intermediately bound') seems to dominate at body temperature with thermodynamic preference. The ligand binding studies were supplemented by aggregation studies via dynamic light scattering and bioinformatic analyses. Beyond the remarkably fine-tuned binding properties exhibited by each FABP, which were discernible with our EPR-centered approach, the results of this work attest to the power of simple spectroscopic experiments to provide new insights into the ligand binding mechanisms of proteins in general on a molecular level.

Macromolecular and Solution Properties of the Recombinant Fusion Protein HUG

The recombinant fusion protein HELP-UnaG (HUG) is a bifunctional product that exhibits human elastin-like polypeptide (HELP)-specific thermal behavior, defined as a reverse phase transition, and UnaG-specific bilirubin-dependent fluorescence emission. HUG provides an interesting model to understand how its two domains influence each other’s properties. Turbidimetric, calorimetric, and light scattering measurements were used to determine different parameters for the reverse temperature transition and coacervation behavior. This shows that the UnaG domain has a measurable but limited effect on the thermal properties of HELP. Although the HELP domain decreased the affinity of UnaG for bilirubin, HUG retained the property of displacing bilirubin from bovine serum albumin and thus remains one of the strongest bilirubin-binding proteins known to date. These data demonstrate that HELP can be used to create new bifunctional fusion products that pave the way for expanded technological applications.

Frataxin deficiency in neonatal rat ventricular myocytes targets mitochondria and lipid metabolism

Friedreich ataxia (FRDA) is a hereditary disease caused by deficient frataxin expression. This mitochondrial protein has been related to iron homeostasis, energy metabolism, and oxidative stress. Patients with FRDA experience neurologic alterations and cardiomyopathy, which is the leading cause of death. The specific effects of frataxin depletion on cardiomyocytes are poorly understood because no appropriate cardiac cellular model is available to researchers. To address this research need, we present a model based on primary cultures of neonatal rat ventricular myocytes (NRVMs) and short-hairpin RNA interference. Using this approach, frataxin was reduced down to 5 to 30% of control protein levels after 7 days of transduction. At this stage the activity and amount of the iron-sulfur protein aconitase, in vitro activities of several OXPHOS components, levels of iron-regulated mRNAs, and the ATP/ADP ratio were comparable to controls. However, NRVMs exhibited markers of oxidative stress and a disorganized mitochondrial network with enlarged mitochondria. Lipids, the main energy source of heart cells, also underwent a clear metabolic change, indicated by the increased presence of lipid droplets and induction of medium-chain acyl-CoA dehydrogenase. These results indicate that mitochondria and lipid metabolism are primary targets of frataxin deficiency in NRVMs. Therefore, they contribute to the understanding of cardiac-specific mechanisms occurring in FRDA and give clues for the design of cardiac-specific treatment strategies for FRDA.

Direct determination of unbound lipophilic ligands in aqueous solutions

Due to their hydrophobic nature, lipophilic compounds are always bound to proteins when transported in the organism. The transfer of such compounds between their binding proteins and cells as well as intracellular trafficking is mediated by a very low water-phase concentration of monomers. The use of protein filled resealed red cell membranes (erythrocyte ghosts) as semipermeable bags enables us to determine directly such water-phase concentrations in a biological system where the lipophilic compound is in equilibrium with the compound bound to its binding protein. Equilibrium dissociation constants (K(d)'s) and number of binding sites are determined by regression analyses of data. We describe the method with the hydrophobic anion arachidonate and the neutral N-arachidonoylethanolamide as examples.

Structural changes accompanying human serum albumin's binding of fatty acids are concerted

Long chain fatty acids (LCFAs), a major source of cellular energy, are solubilized and transported in the blood by binding to serum albumin. Changes in human serum albumin's (HSA's) UV absorption and characteristic reactivity with pyridoxal-5'-phosphate appear to reflect a concerted change in its structure upon binding five equivalents of myristate. Isothermal titrations with myristate and other LCFA anions are also consistent with the presence of five strong, interacting, binding sites. Although HSA is usually thought to have many independent LCFA anion binding sites, just five interacting sites appear to account for the changes in structure that accompany its binding of myristate.

Perinatal changes in myocardial metabolism in lambs

BACKGROUND

Lactate accounts for a third of myocardial oxygen consumption before and in the first 2 weeks after birth. It is unknown how the remainder of myocardial oxygen is consumed. Glucose is thought to be important before birth, whereas long-chain fatty acids (LC-FA) are the prime substrate for the adult. However, the ability of the myocardium of the newborn to use LC-FA has been doubted.

METHODS AND RESULTS

We measured the myocardial metabolism of glucose and LC-FA with [U-(13)C]glucose and [1-(13)C]palmitate in chronically instrumented fetal and newborn lambs. In fetal lambs, myocardial oxidation of glucose was high and that of LC-FA was low. Glucose and LC-FA accounted for 48+/-4% and 2+/-2% of myocardial oxygen consumption, respectively. In newborn lambs, oxidation of glucose decreased, whereas oxidation of LC-FA increased. Glucose and LC-FA accounted for 12+/-3% and 83+/-19% of myocardial oxygen consumption. To test whether near-term fetal lambs could use LC-FA, we increased the supply of LC-FA with a fat infusion. In fetal lambs during fat infusion, the oxidation of LC-FA increased 15-fold. Although the oxidation of LC-FA was still lower than in newborn lambs, the contribution to myocardial oxygen consumption (70+/-13%) was the same as in newborn lambs.

CONCLUSIONS

These data show that glucose and lactate account for the majority of myocardial oxygen consumption in fetal lambs, whereas in newborn lambs, LC-FA and lactate account for the majority of myocardial oxygen consumption. Moreover, we showed that the fetal myocardium can use LC-FA as an energy substrate.

Effect of albumin administration on L-tryptophan levels in the serum and tissues of rats with puromycin aminonucleoside-induced nephrosis

We examined the effect of albumin administration on L-tryptophan (Trp) levels in the serum and tissues of rats with puromycin aminonucleoside (PAN)-induced nephrosis. PNA-injected rats showed increased urinary protein and serum non-esterified fatty acids levels and decreased serum albumin level at 8 days after the injection. The nephrotic rats showed a decrease in total serum Trp level and increases in free serum Trp and liver and kidney Trp levels but no change in urinary Trp level. At 5 min after intravenous injection of bovine serum albumin, the nephrotic rats had serum albumin concentration similar to the level of non-nephrotic rats and showed a recovery of decreased total serum Trp with the reduction of increased free serum Trp and liver Trp.

Expression of recombinant psoriasis-associated fatty acid binding protein in Escherichia coli: gel electrophoretic characterization, analysis of binding properties and comparison with human serum albumin

The psoriasis-associated fatty acid binding protein (PA-FABP, also known as FABP5) is a novel keratinocyte protein that is highly up-regulated in psoriatic plaques (P. Madsen, H. H. Rasmussen, H. Leffers, B. Honoré and J. E. Celis, J. Invest. Dermatol. 1992, 99, 299-305). Here we have expressed PA-FABP in Escherichia coli as a fusion protein containing an NH2-terminal hexa-His tag followed by a factor Xa cleavage site. The recombinant protein was expressed at a level of about 30% of the soluble proteins and was purified to homogeneity using a simple two-step protocol consisting of affinity chromatography on Ni2+-nitrilotriacetic acid agarose followed by gel filtration. The recombinant protein was then digested with factor Xa and characterized by two-dimensional gel electrophoresis. The ability of PA-FABP to bind saturated fatty acids ranging from 6 to 16 carbons was determined directly by dialysis and compared to human serum albumin (HSA). The results showed that PA-FABP binds multiple molecules of the fatty acids hexanoate (C6:0), octanoate (C8:0), decanoate (C10:0) and laurate (C12:0), all with a K1 of about 10(4) M(-l), and myristate (C14:0) with a K1 of 4.4 X 10(5) M(-l). Palmitate (C16:0) also bound strongly with multiple molecules. Due to the very low solubility of palmitate its affinity to PA-FABP was measured relatively to HSA and found to be 8.1 times lower. At ligand/protein ratios below 1, all fatty acids bound to PA-FABP with about one to three orders of magnitude lower affinity than to HSA. The difference in the fatty acid binding properties of the two proteins may reflect differences in their three-dimensional structures, which in the case of PA-FABP consists mainly of beta-sheets while HSA contains predominantly alpha-helices.

Palmitate and stearate binding to human serum albumin. Determination of relative binding constants

Multiple binding equilibria of two apparently insoluble ligands, palmitate and stearate, to defatted human serum albumin were studied in a 66 mM sodium phosphate buffer (pH 7.4) at 37 degrees C, by determination of dialytic exchange rates of ligands among identical equilibrium solutions. The experimental data were analysed by a computerised curve fitting procedure using equilibrium equations for multiple binding of ligands, containing relative binding constants, valid whether the ligands are truly insoluble or are slightly soluble and irrespective of aggregation in aqueous solution. A best-fit set of relative binding constants was found, and subsequently 30 sets of acceptable constants for each set of data in order to evaluate the variation. The data were first fitted by the relative Scatchard's equation, then by the relative, stoichiometric equation. Scatchard's equation is deduced on the presumption that cooperativity is absent while the stoichiometric equation is valid even when cooperativity is present. It was found with palmitate as well as with stearate that the two equations fitted the data equally well, and it was concluded that the observations were compatible with absence of cooperativity. The relative Scatchard binding constants were converted to relative, stoichiometric constants and it was found that the variations of the latter were slight.

Improved electrochemical properties of stearate-graphite paste electrodes after albumin and phospholipid treatments

Stearate-graphite paste electrodes (SGEs) exhibit enhanced dopamine sensitivity and insensivity to asorbic acid electrocatalytic effects in vitro following exposure to unidentified contituents of rat brain tissue homogenates. The present study utilized voltammetry and chronamperometry to compare the electrochemical characteristics of brain-treated SGEs to those treated with potential brain constituent candidates (albumin proteins and phospholipids). Albumin treatments markedly attenuated interference from ascorbate catalytic effects whereas lipids enhanced both electrode capacitance and sensitivity to dopamine. Combined treatments resulted in electrochemical properties that were similar to brain-treated SGEs. Potential mechanisms by which albumin may attenuate ascorbate electrocatalysis of dopamine were investigated using high performance liquid chromatography, with electrochemical detection. The reduction in ascorbate electrocatalytic effects at albumin-treated SGEs may be due to nucleophilic binding of dopamine oxidation products to albumin attached to the electrode surface. Therefore, the unambiguous detection of dopamine by SGEs in vivo may be related to interactions with factors in brain having similar surface-modifying properties.

Effects of ionic strength and pH on the binding of medium-chain fatty acids to human serum albumin

Binding equilibria for the interactions of the medium-chain fatty acid anions, laurate and myristate, with defatted human serum albumin have been investigated under varying environmental conditions such as ionic strength and pH. Since these ligands bind strongly to albumin (Kass approximately 10(7) M-1), conventional equilibrium dialysis is not a feasible method for these investigations. Consequently, we employed a dialysis method, allowing determination of very low concentrations of unbound ligand by measuring the rate of exchange of labelled ligand across a dialysis membrane under conditions of chemical equilibrium. Over a range of ionic strength, 8-68 mM, the binding of the first few molecules of laurate to albumin was weakened with increasing ionic strength, whereas the binding of subsequent molecules seemed to proceed independently of ionic strength. The binding of myristate by albumin, however, appeared to be independent of ionic strength in the observed range of concentrations. The influence of pH in the range 5.1-9.0 on the binding of the two fatty acid anions by albumin was more complicated. The first molecule of laurate appeared to bind with a slightly weaker affinity to albumin at low pH, compared to pH 7 and high pH, while the trends for the following molecules varied. The binding of myristate (irrespective of concentration) seemed to strengthen monotonously with pH, but this conclusion depends critically on the interpretation of the kinetic behaviour of the myristate anion. We have previously shown [Pedersen, A. O., Honoré, B. & Brodersen, R. (1990) Eur. J. Biochem. 190, 497-502] that the strength of binding of the first few molecules of the two fatty acid anions to albumin decreases with increasing temperature, whereas binding of subsequent molecules seems to proceed independently of temperature. We explain these findings as follows. The binding of the first few (3 or 4) molecules of the C12 laurate anion is clearly driven by formation of ionic bonds between the fatty acid anion and positively charged groups, such as lysine residues, in the albumin molecule, whereas the binding of subsequent molecules of laurate seems to depend more on hydrophobic interactions. In the case of the C14 myristate anion, the binding of the first few (only 1 or 2) molecules may depend on ionic forces, but binding of the following molecules of myristate seems to depend on hydrophobic interactions only.(ABSTRACT TRUNCATED AT 250 WORDS)

Fatty acids in cell signalling: modulation by lipid binding proteins

Long-chain fatty acids and several of their metabolites have now been shown to be involved as primary or secondary messengers in specific cell signalling pathways. In view of their extremely low aqueous solubility, the extracellular as well as intracellular transport of these compounds is assumed to be facilitated by specific lipid binding proteins, such as cytoplasmic fatty acid-binding protein (FABP). In this paper a survey is given on the biological significance and possible modulatory action of intracellular lipid binding proteins for fatty acid-mediated signal transduction pathways.

Long-chain fatty acid-binding to albumin: re-evaluation with directly measured concentrations

In studies on uptake of fatty acids (FA) into organs, the unbound (or free) fatty acid fraction is commonly calculated from the concentration bound to albumin and from published binding constants. However, there is some dispute on the methods used for determining those binding constants. We developed a method allowing direct measurement of unbound FA by extending the previous studies of Svenson et al. [1] and Reed et al. [2]. Albumin was coupled to a solid phase (Sepharose 4B), loaded with FA and equilibrated with an aqueous solution. Laurate, palmitate and oleate concentrations in the aqueous phase were determined at different molar ratios of FA to albumin (r) and at different temperatures. FA albumin-binding constants (Ki) increase with chain length and decrease with temperature, in accordance with data obtained by others. However, the unbound concentrations measured are markedly lower than those obtained from binding constants, and the resulting Ki values markedly higher. This difference is presumed to result from (1) our direct measurement of unbound FA and (2) utilizing different more physiological conditions. Recalculating kinetic parameters from published FA uptake data, we found considerably different Km and Vmax values compared to the original data. Thus, the FA-binding characteristics measured in this study may influence the interpretation of FA uptake substantially.

The regulation of fatty acid chain elongation in rat liver microsomes: role of fasting and CoASH

The elongation system for palmitic acid in rat liver microsomes was decreased to 1/3 by fasting, while the elongation of eicosapentaenoic acid was not sensitive to fasting. The rate of eicosapentaenoic acid elongation in the fed state was 50% higher than using palmitic acid as a substrate. The saturated and polyunsaturated fatty acyl-CoA substrates exhibited positive cooperativity on the rate-limiting condensing step in the elongation system, with a Hill constant of approx. 2. An inhibition by CoASH on the total elongation reaction as well as on the condensation step was demonstrated using acyl-CoA substrates, and followed a hyperbolic pattern. The concentrations giving a 50% inhibition (30-70 microM) were in the range found in rat hepatocyte cytosol, indicating that free CoASH has the potential to act as a physiological regulator.

Measurement of palmitate availability in serum samples: method and utility

Palmitic acid shows a very low and unknown solubility at neutral pH. Binding equilibria of palmitate to human serum albumin accordingly cannot be investigated by measuring free and bound ligand concentrations as in conventional binding studies. It is feasible, on the other hand, to describe the binding equilibria in relative terms, by measuring the concentration, p, of reserve albumin, previously defined as the concentration of a purified standard albumin preparation which in buffered solution binds a trace amount of palmitate as tight as it is bound in the sample. Palmitate availability is calculated as C/p, when C is the concentration of bound palmitate. The general binding equation is modified to contain the availability beside relative binding constants, Li = Ki/K1,St, where K1,St is the first stoichiometric binding constant for palmitate to the standard albumin preparation. Availabilities and relative binding constants can replace free concentrations and usual binding constants in considerations of biochemical transport and enzymatic mechanisms. A method is described for measuring the concentration of reserve albumin for binding of palmitate, based upon determination of dialytic exchange rates for palmitate among identical equilibrium samples. A technique for reproducibly adding radiolabelled palmitate to the samples is given.

Valproate and palmitate binding to serum albumin in valproate-treated patients. Relation to obesity

Binding of valproate and palmitate to serum albumin was studied in 29 valproate-treated epileptic patients. The results were compared with similar observations in a reference group of 43 non valproate-treated individuals. The binding affinity for palmitate was decreased (P < 0.0001) resulting in increased availability of long-chain fatty acids (P = 0.008) due to competitive valproate binding in the valproate-treated patients. The findings support a hypothesis on the pathogenesis of obesity as a complication of valproate treatment of epilepsy.

Release of fatty acid-binding protein and long chain fatty acids from isolated rat heart after ischemia and subsequent calcium paradox

To obtain insight into the relation between the release of heart-type fatty acid-binding protein (H-FABPc) and of long-chain fatty acids (FA) from injured cardiac tissue, rat hearts were Langendorff perfused according to the following scheme: 30 min normoxia, 60 min ischemia, 30 min reperfusion, 10 min Ca2+ free perfusion and finally 10 min Ca2+ repletion. During this protocol right ventricular (Qrv) and interstitial effluent samples (Qi) were collected at regular intervals. During reperfusion a total of 0.8 +/- 0.1 nmol H-FABPc but no FA were detected in the effluents. However, during Ca2+ readmission, 45 +/- 4 nmol H-FABPc (80-90% of total tissue content) was released with an initial (first 3 min) simultaneous release of FA (FA/H-FABPc ratio 0.90 +/- 0.07 mol/mol). Thereafter, FA release continued at 10-15 nmol per min mainly in Qrv while the rate of H-FABPc release decreased. During Ca2+ repletion, tissue FA content raised rapidly from 168 +/- 20 to 1918 +/- 107 nmol/g dry weight. These findings suggest that after severe cardiac damage initially FA is released bound to H-FABPc, whereas further FA release occurs in a non-protein bound manner.

Biothermodynamic characterization of monocarboxylic and dicarboxylic aliphatic acids binding to human serum albumin: a flow microcalorimetric study

Thermodynamic parameters have been evaluated for the binding of unbranched monocarboyxlic aliphatic acids (MCAs) of 4 to 16 carbons (MC4 to MC16) and dicarboxylic aliphatic acids (DCAs) of 4 to 16 carbons (DC4 to DC16) to human serum albumin (HSA) on the basis of microcalorimetric measurement at pH 7.4 and 37 degrees C by computer-fitting to single- and two-class binding models. Long-chain MCAs (MC10 to MC16) and DCAs (DC14 and DC16) had the first class of binding sites with high affinity (large binding constant) of 10(5) to 10(6) M-1 and the second class with lower affinity and high capacity (large numbers of binding sites). Short- or medium-chain MCAs and DCAs bound to HSA at some low affinity binding sites. The binding constants of MCAs were ten times larger than those of DCAs. All the relationships between the thermodynamic parameters and alkyl-chain length of the acids showed clear-cut inflections in their plots around eight or nine methylene units. The free energy change of the first class of binding sites (- delta G1) became more negative with an increment of -1.0 kJ mol-1 CH2(-1) as the alkyl-chain length increased, but there were steep rises between MC9 and MC11 with -2.90 kJ mol-1 CH2(-1) and between DC9 and DC12 with -2.02 kJ mol-1 CH2(-1). The enthalpy change (- delta H) increased at the rate of -7.4 kJ mol-1 CH2(-1) to the maximum at MC9 and DC10, then decreased due to hydrophobicity of the alkyl-chains. From compensation analyses (delta H vs. delta S and delta G), HSA binding sites were characterized into three groups.

Cytoplasmic fatty acid binding protein: significance for intracellular transport of fatty acids and putative role on signal transduction pathways

The cellular transport of long-chain fatty acid moieties is thought to be mediated by a plasmalemmal and a cytoplasmic fatty acid binding protein (FABPPM and FABPC, respectively) and a cytoplasmic acyl-coenzyme A binding protein (ACBP). Their putative main physiological significance is the assurance that long-chain fatty acids and derivatives, either in transit through membranes or present in intracellular compartments, are largely complexed to proteins. FABPC distinguishes from the other proteins in that distinct types of FABPC exist and that these are found in a variety of tissues in remarkable abundance, with some cells containing more than one type In addition, liver type FABPC binds not only fatty acids, but also several other hydrophobic ligands, including heme, bilirubin, prostaglandin E1 and lipoxygenase metabolites of arachidonic acid. Calculations made for rat cardiomyocytes reveal that the presence of FABPC substantially enhances the cytoplasmic solubility as well as the maximal diffusional flux of fatty acids in these cells. Apart from this putative function in the bulk transport of ligands, FABPC may also function in the fine-tuning of cellular events by modulating the metabolism of hydrophobic compounds implicated in the regulation of cell growth and differentiation.

Exchange efflux of [3H]palmitate from human red cell ghosts to bovine serum albumin in buffer. Effects of medium volume and concentration of bovine serum albumin

[3H]Palmitate, PA, exchange efflux kinetics is recorded from human erythrocyte ghosts to buffer with bovine serum albumin, BSA, at 0 degrees C. The effects have been investigated of three medium/ghost volume ratios: 36, 80 and 500, of six BSA concentrations, [BSA]: 0.01, 0.02, 0.05, 0.2, 1 and 2% (1.5, 3.0, 7.5, 30, 150 and 300 microM) and of various v, molar ratios of palmitate to BSA, between 0.15 and 0.94. Data are analyzed in terms of a virtually closed three-compartment model. In theory, the tracer efflux is biexponential and the rate coefficients differ at least 20 fold [1]. The efflux rate at 2% BSA is monoexponential beyond our resolution time of about 1 s, but nearly biexponential at or below 0.2% BSA with a well-defined smallest-rate coefficient beta. beta depends strongly on [BSA] but is remarkably v independent. The medium/ghost volume ratio has no effect on beta when [BSA] > or = 0.2%, although beta measured at 2% BSA is almost 2-fold higher than at 0.2%. This suggests the presence of an unstirred layer, USL. According to our model, the observations are understood quantitatively on basis of our previously published dissociation rate constants of the PA-BSA complex, as well as PA equilibrium bindings to ghost membranes (Bojesen, I.N. and Bojesen, E. (1991) Biochim. Biophys. Acta 1069, 297-307). Essentially, beta is theoretically a function of two terms, one comprising the membrane transport parameters and the other the medium-dependent variables. Most important is the clearance with respect to monomer concentration adjacent to the membrane. The clearance is calculated on basis of quasi-stationary diffusion in USL. The data are compatible with a planar USL of 6 microns depth and with the same area as a ghost but not with a spherical USL.

Fatty acid and drug binding to a low-affinity component of human serum albumin, purified by affinity chromatography

Binding equilibria for decanoate to a defatted, commercially available human serum albumin preparation were investigated by dialysis exchange rate determinations. The binding isotherm could not be fitted by the general binding equation. It was necessary to assume that the preparation was a mixture of two albumin components about 40% of the albumin having high affinity and about 60% having low affinity. By affinity chromatography we succeeded in purifying the low-affinity component from the mixture. The high-affinity component, however, could not be isolated. We further analyzed the fatty acid and drug binding abilities of the low-affinity component. The fatty acids decanoate, laurate, myristate and palmitate were bound with higher affinity to the mixture than to the low-affinity component. Diazepam was bound with nearly the same affinity to the low-affinity component as to the albumin mixture, whereas warfarin was not bound at all to the low-affinity component.

Solubility of long-chain fatty acids in phosphate buffer at pH 7.4

The solubility of the saturated fatty acids lauric, myristic, palmitic, and stearic acid and the unsaturated oleic acid at 37 degrees C in phosphate buffer (pH 7.4) was estimated by using two independent methods. The one was a conventional solubility technique measuring the concentration of dissolved fatty acid in buffer by using radioactive compounds. The other was a dialysis exchange technique monitoring possible aggregation of solvated fatty acid anions by measuring the rate of diffusion of labelled compound across a dialysis membrane under conditions of chemical equilibrium. It was found that the results were strongly dependent on the radiochemical purity of the fatty acids. Using highly purified samples of radioactively labelled fatty acids, the solubility of monomeric laurate was shown to be greater than 500 microM, whereas the solubility of monomeric myristate was found to be 20-30 microM. Palmitate, stearate, and oleate solutions, on the other hand, showed a tendency to aggregation even at concentrations below 1 microM. Special attention was given to palmitate, as a reference compound for long-chain fatty acids, and the solubility of monomeric palmitate was estimated to be lower than 10(-10) M.

Binding of long-chain fatty acids to serum albumin in healthy humans. Relationship to obesity

Equilibria of the binding of palmitate to serum albumin in adults are studied by dialysis-exchange-rate determinations. The results are used for a description of binding equilibria of fatty acids in general, as follows. 1. The reserve albumin concentration, p, for binding of palmitate is used as an approximate measure of p*, the reserve albumin concentration for binding of mixed fatty acids present in serum. 2. The total availability of fatty acids is defined as C*/p*, where C* is the total concentration of non-esterified fatty acid. 3. The fatty-acid-binding property of albumin is described by L* = p*/P = alpha C*/P, where P is the albumin concentration. The numerical value of alpha is -0.05. The above parameters are measured in sera from four healthy volunteers, in whom large variations of serum fatty acid concentration occurred. A group of 64 healthy students showed considerable variation of L* from one individual to another. It is found that L* decrease significantly with increasing body mass index (body mass divided by the square of the body length). In 42 patients with diabetes type I, L* was independent of body mass index. These findings are consistent with a previously formulated hypothesis of mechanism of obesity.

Fatty acid binding to serum albumin in type I insulin-dependent diabetes mellitus

Equilibria of binding of long-chain fatty acids to albumin in sera from type I diabetic patients and healthy adults were studied by dialysis exchange rate determinations and described by, p*, the reserve albumin concentration for binding of fatty acid, C*/p*, the total availability of fatty acids, where C* is the total concentration of non-esterified fatty acid, and L*, the fatty acid binding property of albumin, which is L* = p*/P + 0.05 C*/P, where P is the albumin concentration. Studies in samples from 81 diabetic patients and 99 healthy adults showed that availability of fatty acids increased with increasing fatty acid concentrations, equally in the two groups. Some diabetics had higher fatty acid concentrations, and thus higher fatty acid availabilities, than the normals. It is shown that the fatty acid binding property of serum albumin is individually variable, ranging about the same mean value in normal and diabetic persons but with a larger variation in the latter. The fatty acid binding property of albumin in serum, L*, and sixteen clinical parameters were measured in 42 of the 81 diabetic patients. Regression analysis indicated that L* was correlated to serum cholesterol concentration (probability of 0-hypothesis, p = 0.01) and to serum triglyceride concentration (p = 0.05). Values of L* were slightly correlated to age, age on diagnosis, duration, Body Mass Index (BMI), diastolic blood pressure, albumin excretion rate, serum creatinine concentration, and serum non-esterified fatty acid concentration with p-values varying from 0.10 to 0.50. For sex, retinopathy, hemoglobin A1c, systolic blood pressure, daily insulin dose, and blood glucose concentration no correlation to L* was found, p-values ranging from 0.56 to 0.96. Non-enzymatic glycosylation of serum albumin did not decrease binding affinity for fatty acid in vitro.

Cellular fatty acid-binding proteins: current concepts and future directions

At least three different proteins are implicated in the cellular transport of fatty acid moieties: a plasmalemmal membrane and a cytoplasmic fatty acid-binding protein (FABPPM and FABPC, respectively) and cytoplasmic acyl-CoA binding protein (ACBP). Their putative main physiological significance is the assurance that long-chain fatty acids and derivatives, either in transit through membranes or present in intracellular compartments, are largely complexed to proteins. FABPC distinguishes from the other proteins in that distinct types of FABPC are found in remarkable abundance in the cytoplasmic compartment of a variety of tissues. Although their mechanism of action is not yet fully elucidated, current knowledge suggests that the function of this set of proteins reaches beyond simply aiding cytoplasmic solubilization of hydrophobic ligands, but that they can be assigned several regulatory roles in cellular lipid homeostasis.

Thermodynamic parameters for binding of fatty acids to human serum albumin

Binding of laurate and myristate anions to human serum albumin has been studied over a range of temperatures, 5-37 degrees C, at pH 7.4. The binding curves indicate that the strength of binding of the first few molecules of fatty acid to albumin (r less than 5) decreases with increasing temperature, whereas binding of the following molecules seems to proceed independently of temperature. Binding data were analyzed according to the general binding equation yielding several sets of acceptable binding constants within a probability limit of 0.75. From the temperature dependence of the first step constant, it was possible to calculate values for the changes in enthalpy and entropy during the initial binding step. For the medium-chain fatty acids, laurate and myristate, binding of the first molecule to albumin appeared to be enthalpic, with a tendency to an increasing contribution of entropy to binding energy with increasing chain length of the fatty acid.

Multiple fatty acid binding to albumin in human blood plasma

Binding equilibria of long-chain fatty acids to human serum albumin, in serum or plasma, were studied by a dialysis exchange rate technique. Palmitate was added to citrated plasma in vitro and it was observed that between six and ten palmitate molecules were bound to albumin with nearly equal affinity. Observations in vivo gave similar results in the following series: (a) in two volunteers with increased fatty acid concentrations after fasting, exercise, and a cold shower: (b) in three male volunteers in whom high concentrations of non-esterified fatty acids, up to 4.6 mM, were induced by intravenous administration of a preparation of lecithin/glycocholate mixed micelles, and (c) in 81 patients with diabetes mellitus, type I. The binding pattern of palmitate in serum or plasma is essentially different from that observed with palmitate added to buffered solutions of pure albumin when two molecules are tightly bound and about four additional molecules with lower affinity. The differences may partly be explained by the presence of chloride ions in blood plasma, reducing the affinity for binding of the first two fatty acid molecules, and partly by facilitated binding of several molecules of mixed fatty acids, as found in plasma.